FACTORS INFLUENCING BACTERIAL GROWTH 15 



was the first to note this extraordinary fact and he suggested the 

 grouping of bacteria into two divisions, viz. : aerobes which re- 

 quire the presence of free oxygen, and anaerobes, which require the 

 exclusion of free oxygen. Midway between the obligatory aerobes 

 and the obligatory anaerobes, however, there are many organisms 

 which do not belong strictly to one group or the other. Faculta- 

 tive anaerobes grow best in the presence of oxygen but their growth 

 is not checked when the supply is limited. Facultative aerobes 

 on the other hand are anaerobes that can tolerate a certain amount 

 of free oxygen. Anaerobes are not affected by the presence of 

 hydrogen or nitrogen. 



Light. The effect of light upon bacteria is very marked. 

 Bright daylight may inhibit their growth, and many species can- 

 not live when exposed to the full action of the sun's rays. A 

 longer exposure is necessary when they are moist than when 

 they are dry. Typhoid bacilli are killed in about one and a 

 half hours. The bactericidal effect of light is due mainly to the 

 green, violet, and ultraviolet rays. An interesting experiment 

 illustrating the effect of light upon bacteria may be carried out by 

 pouring inoculated media into a Petri dish, the cover of which has 

 been partly shaded by pasting on a strip of black paper. The 

 plate after being exposed to direct sunlight will show colonies only 

 in the shaded portion. A series of plates may be prepared and 

 exposed varying lengths of time, half an hour, one hour, etc. 

 The plates should be kept two or three days at 20 C. to 30 C. 

 to allow the colonies in the shaded portion to develop. 



Electricity. The effects of electricity upon bacteria have not 

 been thoroughly studied. A powerful electric light is supposed 

 to be as fatal as sunlight. 



Radium and Rontgen rays have not been shown to have more 

 than a slight inhibitory action. 



Temperature. The range of temperature within which bac- 

 terial growth of one species or another may occur lies between C. 

 and 72 C. For every species there is an optimum temperature 

 or a temperature at which its growth is most luxuriant. Each 

 species too has its maximum temperature above which growth 



